a) Field of the Invention
This invention relates to an anti-skid braking method suited for application to brakes of an automotive vehicle. This invention is also concerned with an anti-skid braking system suited for the practice of the anti-skid braking method.
b) Description of the Related Art
An anti-skid braking method is known, which upon braking on a low-.mu. road such as a rain-wet road, can prevent slipping of wheels, retain steerability and allow a vehicle to stop in a short stopping distance. According to this known braking method, the revolution speed of each wheel is detected to determine the speed of the wheel. Based on the difference between the speed of the wheel and a reference speed of the vehicle body, the slip ratio of the wheel is determined. Further, time-dependent variations of the speed of the wheel, namely, the wheel acceleration is also determined. In accordance with the slip ratio and wheel acceleration so determined, braking pressure for the wheel is then increased or decreased so that the slip ratio can be maintained near an optimal slip ratio at which the coefficient of friction of the wheel becomes the greatest.
The wheel acceleration can be used as a control parameter for determining whether the wheel is controlled toward a locked state (that is, tends to lock) or is controlled toward a brake-released state (that is, tends to be released from the brake) by the increase/decrease control of the brake pressure.
Upon running on an extremely low .mu.-road having a very small coefficient of friction, each wheel may be judged to be in a slightly decelerated state because of an acceleration of the wheel although the slip ratio of the wheel is greater than an optimal control range. The hydraulic brake pressure may therefore neither decrease nor increase or, even when controlled, is decreased only slightly. If such a state is permitted to remain as is without any control, the wheel is brought toward a locked state and eventually is locked. Such a state of the wheel, in which the wheel is gradually brought toward a locked state, involves the problem that it cannot be exactly detected as long as only the acceleration of the wheel is relied upon as in the conventional art.
When an automotive vehicle advances from a low-.mu. road having a low coefficient of friction to a high-.mu. road such as an asphalted road, the conventional increase/decrease control of the hydraulic brake pressure has a slow rise in longitudinal acceleration and also has difficulty in accurately detecting the advance to the high-.mu. road on the basis of a wheel acceleration. The conventional control is therefore accompanied by the problem that the pressure increase immediately after the advance is delayed and the application of brakes is delayed accordingly. Because of the slow rise in longitudinal acceleration, the driver feels as if the brakes are not effectively applied, in other words, the driver feels a so-called "idle-running". This is certainly undesirable.